1. Field of the Invention
The present invention relates to high power microwave sources. More specifically, the present invention relates to apparatus for compressing microwave pulses to produce high power pulses of selectable shape and power, including a laser induced switch.
2. Description of Related Art
Microwaves are electromagnetic waves whose frequencies range from approximately 300 megahertz (MHz) to 1000 gigahertz (GHz). The lower end of the microwave region is bounded by radio and television frequencies, and the upper end is bounded by infrared and optical spectrums. Most applications of microwave technology make use of frequencies in the 1 to 40 GHz range, including the bands labeled the L band, S band, C band, X band, K.sub.u band, K band, and K.sub.a band.
Microwave energy has many applications. For example, point-to-point communications and radar advantageously use microwave energy due to its ability to be focused better than lower frequencies. The heating properties of microwave radiation are well known; the microwave oven has become a standard kitchen fixture. Medical applications are possible, for example, the possibility of exposing malignant cells to microwave radiation is being investigated as a method of treating cancer. Other microwave applications include material science. For example, many substances exhibit atomic and molecular resonances when exposed to microwave radiation. The study of these resonances is called microwave spectroscopy, and is of importance in the scientific effort to understand the fundamental nature of solids, liquids, and gases. High-energy particle physics also utilize microwave techniques as, for example in particle accelerator developments.
For military applications, it is sometimes useful to characterize a component by its resistance to directed microwave radiation. To properly characterize a component, it must be tested by high power microwave pulses.
A limitation on the known sources of microwave radiation is the combined lack of power and control. Some sources can produce high power, and others can produce a reliable and controllable pulse. However, some applications require high peak power, (for example 1 gigawatt (Gw)), high repetition rates (&gt;2 pulses per second (pps)), with highly controlled pulse shapes. Also, a quick rise time (a fast pulse) and a short duration is often desirable.
There are many examples of well known sources of microwave radiation, such as klystrons and magnetrons.
Klystrons can produce a microwave pulse that is stable in power, duration, and wavelength at repetition rates up to the KiloHertz level. The duration of the klystron pulse is long, for example 150 nanoseconds, and has a rise time that is long, for example 70 nanoseconds. In the pulsed mode of operation, power may reach a peak of 20 megawatts.
Another source of microwave radiation is the magnetron tube. Magnetrons are a type of cross-field microwave electron tubes wherein electrons, generated from a heated cathode, move under the combined force of a radial electric field and axial magnetic field. Magnetrons have been used since the 1940s as pulsed microwave radiation sources for radar equipment in aircraft as well as ground radar stations. Magnetrons can be used for high power applications where the noise is not a problem. In continuous operation, a magnetron can be suitable for microwave cooking. A typical magnetron system may produce a peak power up to a hundred megawatts, with a low repetition rates. The typical magnetron produces a long pulse with a long rise time.
An exemplary high power relativistic magnetron is capable of providing a noisy 10 gigawatt (GW) pulse for high field, high peak power testing of components and systems. The exemplary microwave pulse has a width of 20-50 nanoseconds, during which unpredictable power spikes may occur. This duration is relatively long, and the risetime is very slow, such as half of the pulse duration. Furthermore, the repetition rate is only one per several hours, and from pulse to pulse, the output frequency may vary unpredictably.
In order to provide high power pulses of short duration, pulse compression methods have been developed to compress a longer, low power pulse into a shorter time, thereby providing higher power. For example, a one microsecond pulse may be compressed to 4 nanoseconds, a decrease of a factor of 250. The peak power of the pulse may increase correspondingly, by a factor of about 100.
Several systems have been developed to compress microwave pulses. In one pulse compression system, microwave energy from a conventional source such as a klystron tube is coupled into a microwave cavity. This cavity comprises a microwave waveguide having a rectangular shape with a lateral branch forming a "T" section. One end of the man cavity has a coupler for coupling n microwave energy, and the other end of the cavity is terminated in a short circuit. Microwave energy is coupled into the main cavity, and initially, the microwave energy coming into the main cavity is stored, and leakage through the lateral branch is low. After the amount of stored microwave energy has increased to the desired level, a switch is actuated, and the microwave energy exits the main cavity through the lateral branch. An explanation of the switching operation is provided in U.S. Pat. No. 4,227,153, issued Oct. 7, 1980 to Birx, which is incorporated by reference herein.
The switch for the microwave cavity may comprise a gas plasma discharge switch, positioned at a distance either a quarter of a guide wavelength (.lambda.g) from the shorted end of the cavity, or odd multiples of a quarter of a guide wavelength. The switch described in U.S. Pat. No. 4,227,153 includes a discharge tube, which is relatively transparent to microwave energy when it is not actuated. The discharge tube may be filled by neon, for example, and the tube may be actuated by electrodes positioned on either end of the discharge tube. When a voltage is applied across the electrodes, a plasma is formed in the tube, which reflects microwave energy. This reflection effectively changes the character of the main cavity so that the microwave energy is switched out of the main cavity and into the lateral branch within a very short duration.
The U.S. Pat. No. 4,227,153, referenced above, discloses an apparatus that utilizes superconducting elements for generating high power microwave pulses. Specifically, that apparatus includes a superconducting resonator, which is known to maintain very high Qs and therefore it can store a high field strength. The Q value is a measure of the resonator's capacitor for resonance and energy storage at a particular frequency. Because of the high Q, a superconducting resonator can directly store much larger amounts of microwave energy than a resonator made from non-superconducting material, which generally exhibits a much lower Q. The larger amount of energy storage in the superconducting resonator makes it possible to provide a pulse of high power when the discharge switch is actuated.
Another type of switch, similar to the discharge switch, is similarly positioned in the cavity. The spark gap discharge switch comprises a spark gap positioned at the end of the gas tube. When actuated, the spark creates a few free electrons. The microwave energy is coupled to these electrons, and the number of electrons quickly cascades. Within a very short time, a plasma is created in the gas tube, which reflects the microwaves, thereby switching the microwave pulse.
The waveguide with a "T" branch described above can provide a compressed microwave pulse that has a high power, a fast rise time, and a predictable wavelength and pulse shape. However, the repetition rate of the system is limited by the rise time of the electronics necessary to provide the spark. Furthermore, the repetition rate is ultimately limited by the effective recovery rate of the gas discharge tube. Currently the repetition rate is limited to about one pulse per second.
Still another type of switch is described in U.S. Pat. No. 4,255,731, issued Mar. 10, 1981 to Birx, which is incorporated by reference herein. Instead of producing a plasma in an ionizable gas, the switch described in that patent achieves switching by generating an intense beam of electrons in the main waveguide cavity. The intense electron beams are directed transversely to the main waveguide cavity, in a direction parallel to the electric field of the microwave radiation. The patent describes several problems with the gas plasma switch, including its positioning in a region of electrical field intensity which cause losses in the stored energy. Furthermore, rise time is limited by the time needed for the gas of a discharge tube to ionize and produce the appropriate plasma. Furthermore, once the gas is ionized to form a plasma, a certain time for recombination exists during which it is reflective. Until the plasma has adequately decayed, energy storage in the main cavity is limited. The time necessary for effective decay may be termed the recovery time. The repetition rate of the system is ultimately limited by an effective recovery time.
A problem of the gas discharge tube, the spark gap switch, and the intense electron beam switch is their dependence on high voltage electronics. The rise time and the fall time of the electronic systems associated with the switching systems limits the predictability and repetition rates of the output pulses. Typically, the repetition rate may be limited to one pulse per second or less. Furthermore, the output power from a single cavity is limited by the amount of energy that can be stored. Although a substantial amount of microwave energy can be stored in a single cavity, still it is a finite amount, and does not meet many needs of researchers and others in the microwave field. There is a need for a system that can provide microwave pulses having higher power, higher energy, and greater control.
Another problem of the pulse compression systems described above is pre-pulse leakage. Although the amount of leakage is low, still it is a finite amount that is undesirable for some applications. It would be an advantage to have a microwave pulse compression system with lower pre-pulse leakage.